The present invention relates to load sensing hydraulic systems of the type including both a priority load circuit and an auxiliary load circuit, and more particularly, to such systems in which the control valve within the auxiliary load circuit operates by means of a pilot pressure command.
Load sensing hydraulic systems of the type to which the present invention relates are used in many applications, and are especially well suited for use on mobile, off-highway vehicles. By way of example only, on such a vehicle, the hydraulic system may include, as the priority load circuit, a hydrostatic power steering system. Such a system would typically communicate pressurized fluid to either a steering cylinder, to turn the vehicle wheels, or to a set of track motors, to vary the relative speeds of the left and right tracks, and thus turn the vehicle.
The load sensing hydraulic system of the type to which this invention relates would also include, as an auxiliary load circuit, some other hydraulically operated vehicle implement or “work circuit”, such as a winch motor, or an auger motor, or a cylinder to raise and lower an implement, etc. Frequently, the “auxiliary load circuit” would actually comprise several auxiliary load circuits in parallel, each operating a different vehicle implement or work element.
As is well known to those skilled in the art of load sensing hydraulic systems, especially those of the type for use in “mobile” applications, it is greatly preferred to provide only a single source of pressurized fluid (i.e., a single pump) in order to minimize the cost and complexity of the overall vehicle hydraulic system. Unfortunately, as is also well known to those skilled in the art, one result of using only a single source is that the instantaneous demand for pressurized fluid and flow will occasionally exceed the pumping capacity of the single pump supplying the hydraulic circuit, a condition referred to in the art as “saturation” of the system.
In the conventional load sensing hydraulic system of the type which has been in widespread commercial usage, the system includes some sort of load sensing, priority, flow control valve (“LSPV”), to apportion the main, pressurized flow from the single pump between the priority and auxiliary load circuits, in response to a load signal generated by the priority load circuit. An example of such an LSPV is illustrated and described in U.S. Pat. No. 3,455,210, and an example of such a load sensing circuit is illustrated and described in U.S. Pat. No. 4,043,419, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
Load sensing priority flow control valves and systems of the type illustrated and described in the above-cited patents have performed in a generally satisfactory manner in dealing with the above-described problem of the load circuits exceeding pump capacity (“saturation”) and have therefore represented one workable solution to the saturation problem, i.e., the LSPV serves as one possible “anti-saturation” solution. However, because the load sensing priority valve operates on (“throttles”) the main flow path from the pump to the priority and the auxiliary load circuits, the result is a substantial throttling loss, and therefore, a much less efficient overall hydraulic system than is now considered desirable. Furthermore, the inclusion of the typical LSPV in the hydraulic system adds substantially to the overall cost of the system, especially in view of the amount of hydraulic horsepower typically consumed in the LSPV.
Those skilled in the art of load sensing circuits, and especially, the anti-saturation art, will understand that there are various ways of categorizing the potential solutions to the problem. For example, under one of the ways of classifying anti-saturation solutions, the system in which the highest load has its pressure and/or flow reduced (until the pump can satisfy the system's demand), is termed a “pre-compensated” system. Alternatively, under that classification, the system in which all of the loads have their pressures and/or flows reduced proportionally (until the pump can satisfy the system's demand), is termed a “post-compensated” system. However, those skilled in the load sensing art have also used various other definitions to distinguish the terms “pre-compensated” and “post-compensated”, and it should be understood that the present invention is not necessarily limited to use with just one or the other of the types of anti-saturation solutions, regardless of how those terms are defined.